The invention concerns a method for operating a braking device of a drive train with an automatic gearbox of a vehicle, specifically a motor vehicle. Furthermore, the invention concerns a control device for operating a braking device of a drive train with an automatic gearbox of a vehicle, specifically a motor vehicle, as well as a vehicle.
In modern motor vehicles with an automatic gearbox, there are various safety mechanisms to ensure that the vehicle is parked securely. These safety mechanisms forming components of a braking device are primarily the parking brake and the service brake, and also the parking lock of the automatic gearbox itself. The service brake mainly makes it possible to reduce the vehicle speed to make it stop; it is normally operated with a foot via a brake pedal; however, there can also be a brake booster device (active brake booster), or a slip control system—for example, highly-automated and piloted driving. The service brake preferably affects all the wheels of the vehicle. The service brake is often designed as a hydraulic brake, in which the brake power is transferred via liquid pressure in brake lines; however, there can alternatively be a pneumatic service brake, in which the brake power is transferred via compressed air in the brake lines.
Conventionally, the parking brake (often also called emergency brake) is conventionally a mechanical brake that is actuated by the driver's hand or foot, and which creates a locking force in the wheel connected to it via cables. Specifically, when the brake is actuated by the operator, a mechanical force is produced and transferred via mechanical cables to the braking device, in which this force is then exerted onto a brake disk or a brake drum. Modern parking brakes are designed as electric or electronic brakes, in which the actuators that transfer the braking power are controlled by an electric control device. The parking brake is designed to prevent a stopped or parked vehicle from rolling away—even when the vehicle is on a sloped surface. In other words, the parking brake as well as the parking lock, which is described below in more detail, are primarily designed and used for parking a vehicle for an unlimited period of time, whereas the service brake is mainly used to stop the vehicle for a short period of time.
The parking lock in the automatic gearbox works in a way that when the driver switches to the parking gear P, e.g. using a gear lever, a parking catch is engaged in the gearbox, preferably controlled electrically or electromechanically. This parking catch locks the parking interlock gear, which is preferably installed torque-proof on an output drive shaft of the gearbox or the drivetrain. The parking catch can be attached, for example, to the casing of the gearbox. When the parking catch locks the parking interlock gear, the parking interlock gear is blocked in a way that it can no longer turn, which results in the output drive shaft or the drive train of the vehicle being blocked, with the corresponding wheels of the vehicle being locked as well. If a vehicle is parked on a sloping surface, the parking lock is normally subjected to load, as the vehicle (after the parking gear P is engaged) rolls back a little, until the parking lock engages. Specifically, in this case, the parking interlock gear is subjected to torque created by the slope resistance that affects the vehicle. This torque is exerted on the parking lock on the gearbox casing, which leads to the catch gearing to interlock with the gearing of the parking interlock gear, during which (depending on the direction of the torque) a right-side or a left-side parking lock interlock occurs. This interlocked state in the drive train or in the gearbox creates the situation in which, if the parking lock is disengaged or deactivated (i.e. the gearing of the parking catch is moved out of contact with the gearing of the parking interlock gear), the drivetrain is released immediately, which can be felt in the vehicle with a noticeable jerk, being often accompanied by a loud noise (the disengagement impulse), which can be unpleasant or disruptive to the user.
An additional disadvantage is that the force necessary to disengage the parking lock increases with the amount of force exerted to create the interlock. Sometimes this can even create a situation that the parking lock can be disengaged only with a lot of force. The strength of the interlock itself depends on the brake torque of the wheels, which is, in turn, dependent on the slope resistance. The slope resistance depends on the slope of the road surface and the weight of the vehicle.
According to DE 10 2013 212 829 A1, there is already a system and a method acknowledged for combined control of an electronically controlled parking brake and an electronically controlled parking lock, which is designed as a mechanical interlock, of a motor vehicle, which should prevent a disengagement impulse created when disengaging a parking lock of a motor vehicle designed as a mechanical interlock. Specifically, this system comprises an electronic parking device that controls the parking brake electronically, an electronic locking device that controls the parking lock, an electronic hill-start assistant device that controls the parking brake electronically, an electronic device that detects the occurrence of at least one parking condition, and an automatic sequence control device that controls the parking device, the locking device and the hill-start assistant device in a way that, when a parking condition is detected, the parking lock is activated via the locking device, then the parking device begins to increase the braking pressure necessary to lock the parking brake, with the parking brake remaining locked via the hill-start assistant device, until the parking device produces enough braking pressure to lock the parking brake. As a result of the parking brake being locked via the hill-start assistant device until the parking device produces enough braking pressure to lock the parking brake, it should be prevented that a vehicle, after the parking lock is activated, can move from a standing position, when the braking pressure created by the parking device to lock the parking brake is not enough to lock the parking brake. Thus it should be prevented that the parking lock designed as a mechanical interlock comes under stress. Furthermore, this should prevent the unwanted disengagement impulse created when the parking lock is disengaged. This means, specifically, that the vehicle is protected against rolling away only via the parking brake.
Despite this measure, situations can occur when the driver feels an unreduced disengagement impulse. For example, this can happen when the driver (the vehicle being stopped, and the parking lock being engaged) releases the parking brake before disengaging the parking gear: the vehicle can roll away, and thus the parking lock can interlock as described above.